Shots were blasted into the Melbourne office around 3:00am.

The boss of Victorian branch of Australia's main construction union, the CFMEU, has labelled an overnight drive-by shooting a "cowardly act" after a gun was repeatedly fired into his Melbourne office around 3:00am, shattering the front glass door and several upper glass windows.

No one was injured, but staff are said to be "shocked and frightened", after at least four, possibly five, shots were fired by a man into the Swanston Street building.

Penny Stephens/Fairfax
Shots have been fired at the CFMEU HQ in Swanston Street

The gunman fired the shots with what a witness described as a "hunting gun" before driving off.

Victoria Police are investigating with the motive for the shooting unknown at this stage.